This invention is directed to the method of hydraulically fracturing a subterranean formation. More specifically, this invention is directed to a method of forming vertically disposed fractures in a subterranean formation.
Hydraulic fracturing techniques have been extensively used for increasing the recovery of hydrocarbons from subterranean formations. These techniques involve injecting a fracturing fluid down a well and into contact with the subterranean formation to be fractured. Sufficiently high pressure is applied to the fracturing fluid to initiate and propagate a fracture into the subterranean formation. It is generally considered that at depth the fractures that are formed are vertical fractures. This is because at depth the least principal stress in most formations is in the horizontal plane which produces a preferred vertical fracture orientation. Proppant materials are generally entrained in the fracturing fluid and are deposited in the fracture to maintain the fracture open.
Hydraulic fracturing is widely practiced to increase the production rate from oil and gas wells. Fracturing treatments are usually performed soon after the formation interval to be produced is completed, that is, soon after fluid communication between the well and the reservoir interval is established for the purpose of production of injection. Wells are sometimes fractured for the purpose of stimulating production after significant depletion of the reservoir.
Hydraulic fracturing is the principal method used for stimulating production from oil and gas wells in low permeability reservoirs. Almost all of such fractures are vertical. It is always desirable, and sometimes necessary, to limit the vertical extent (height) of such fractures to the hydrocarbon-bearing zone of interest while extending the fracture for a substantial horizontal distance. Frequently, the desired horizontal extent (length) is many times the desired height. The desired result can be readily obtained when the interval to be fractured is bounded above and below by beds which inhibit the growth of fractures, such as soft shales. In many other cases the bounding beds are not effective in inhibiting the vertical growth of fractures. This is a major limitation of application of hydraulic fracturing technology. In such cases the resulting fracture grows into the non-productive bounding beds, and some of the valuable fracturing materials are wasted. In cases where permeable beds containing unwanted fluids, such as water, are also penetrated by the fracture a large amount of unwanted fluid is produced through the fracture into the producing well. In cases where the amount of such unwanted fluid is prohibitive, the well has to be abandoned.